The present invention relates to an audio reproducing apparatus for reproducing a single-channel or a multichannel audio signal by means of headphones.
As a multichannel audio signal, there is known a 5.1-channel digital audio signal employed in the AC-3 (so-called Dolby digital) system. The 5.1-channel digital audio signal S5.1 is generated by encoding the following signals into a piece of serial data.
SLF: audio signal for a channel at the left front of the listener
SRF: audio signal for a channel at the right front of the listener
SCF: audio signal for a channel at the center front of the listener
SLB: audio signal for a channel at the left back (or on the left side) of the listener
SRB: audio signal for a channel at the right back (or on the right side) of the listener
SLFE: audio signal representing a component in a low frequency range below 120 Hz, for example.
The digital audio signal S5.1 recorded on, for example, a DVD together with a video signal provides effects such that when the image of the video signal is reproduced, the position of a sound source in the image coincides with that of a sound image that is actually heard, and also a naturally spreading sound field is created.
The digital audio signal S5.1 is basically intended for reproduction by means of speakers that are disposed around the listener. However, there has been contrived an audio reproducing apparatus that makes it possible to reproduce a similar reproduction sound field by means of headphones.
Specifically, in FIG. 3, reference numeral 200 denotes an example of such an audio reproducing apparatus. Reference numeral 100 denotes a DVD player.
Now, a 5.1-channel digital audio signal S5.1 is extracted from the DVD player 100. The audio signal S5.1 is supplied to a decoder circuit 10 of the audio reproducing apparatus 200, where digital audio signals SLF to SLFE for respective channels are decoded. Then, the decoded and extracted audio signals SLF and SRF are supplied via adding circuits 21 and 22 to a converting circuit 3F, which will be described below, while the audio signals SLB and SRB are supplied via adding circuits 23 and 24 to a converting circuit 3B.
Furthermore, the audio signal SCF is basically a signal to be supplied to a speaker disposed at the center front of the listener; however, a similar effect can be obtained even when the signal SCF is supplied to speakers disposed respectively at the left front and at the right front of the listener. Therefore, the signal SCF is supplied to the adding circuits 21 and 22 to be added to the audio signals SLF and SRF. In addition, the audio signal SLFE is a signal representing a low frequency range below 120 Hz, and its reproduced sound does not provide a directional feeling. Thus, the audio signal SLFE is supplied to the adding circuits 21 to 24 to be added to each of the signals SLF to SRB. Signals SLF0 to SRB0 are extracted from the adding circuits 21 to 24.
The converting circuit 3F is provided to convert audio signals SLF0 and SRF0 into audio signals that can provide a reproduction sound field similar to that obtained in speaker reproduction even when the audio signals SLF0 and SRF0 are to be reproduced by means of headphones. Signal processing by the converting circuit 3F will hereinafter be referred to as virtual sound image localization processing.
Specifically, the converting circuit 3F processes the original audio signals SLF0 and SRF0 in such a way that when output audio signals SLFX and SRFX of the converting circuit 3F are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLF0 and SRF0 before the conversion are supplied to speakers disposed respectively at the left front and at the right front of the listener can be obtained.
Similarly, the converting circuit 3B processes the original audio signals SLB0 and SRB0 in such a way that when output audio signals SLBX and SRBX of the converting circuit 3B are supplied to the headphones, a reproduction sound field similar to that obtained when the audio signals SLB0 and SRB0 before the conversion are supplied to speakers disposed respectively at the left back and at the right back of the listener can be obtained. Incidentally, specific configurations of the converting circuits 3F and 3B will be described later.
Then, an audio signal SLFX from the converting circuit 3F and an audio signal SLBX from the converting circuit 3B are added together at an adding circuit 4L to be converted into a digital audio signal SL for a left channel. Also, an audio signal SRFX from the converting circuit 3F and an audio signal SRBX from the converting circuit 3B are added together at an adding circuit 4R to be converted into a digital audio signal SR for a right channel.
Then, these audio signals SL and SR are supplied to D/A converter circuits 5L and 5R, where the audio signals SL and SR are subjected to D/A conversion to become analog audio signals SL and SR. The resulting audio signals SL and SR are supplied via amplifiers 6L and 6R to acoustic units (electroacoustic transducer devices) 7L and 7R of the headphones 7 for a left and a right channel.
FIG. 4 shows a specific example of the converting circuit 3F. The converting circuit 3F is provided to realize virtual sound image localization processing for the audio signals SLF0 and SRF0 by convolving respective transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener in the audio signals SLF0 and SRF0 using digital filters or by signal processing substantially similar to the above method.
Specifically, the audio signals SLF0 and SRF0 from the adding circuits 21 and 22 are supplied to an adding circuit 31L and a subtracting circuit 31R to form a sum signal and a difference signal. The sum signal and the difference signal are supplied to digital filters 32L and 32R, respectively.
The digital filters 32L and 32R are each provided with a delay circuit 321 having a plurality of stages connected in a cascade manner, a plurality of coefficient circuits 322 and adding circuits 323. The digital filters 32L and 32R are of a FIR type. An input signal is supplied to a first stage of the delay circuit 321 to be sequentially delayed, and an output signal from each stage of the delay circuit 321 is supplied to a coefficient circuit 322 to be multiplied by a specified coefficient. The signals resulting from the multiplication are added to each other by an adding circuit 323, and the result is extracted as a filter output.
Then, output signals from the digital filters 32L and 32R are supplied to a subtracting circuit 33L and an adding circuit 33R to form a difference signal and a sum signal. The difference signal and the sum signal are extracted as output signals SLFX and SRFX of the converting circuit 3F.
Here, respective head related transfer functions for ranges from speakers at the left front and at the right front of the listener to both ears of the listener are convolved in the audio signals SLF0 and SRF0 by providing specified characteristics to the digital filters 32L and 32R. Incidentally, the converting circuit 3B can be configured in the same manner as the converting circuit 3F, and therefore description of the converting circuit 3B will be omitted.
As described above, according to the audio reproducing apparatus 200 of FIG. 3, a reproduction sound field similar to that obtained by means of five speakers disposed at the left front, at the center front, at the right front, in the left rear, and in the right rear of the listener and a speaker for a low frequency range can be reproduced by means of the headphones 7.
However, it has been found that in the case of the audio reproducing apparatus 200 of FIG. 3, reproduced sound outputted from the headphones 7 lacks low frequency sound that should be obtained from the audio signal SLFE representing a component in a low frequency range. According to a consideration given by the inventor of the present invention, it has also been found that the low frequency sound is lacking for the following reasons.
Now, as shown in FIG. 5, if a speaker SP30 is disposed at a position 30xc2x0 to the right of the front of a dummy head DHD, and frequency characteristics when the left ear and the right ear (microphones) of the dummy head DHD receive sound reproduced by the speaker SP30 are measured, characteristics as shown in FIG. 6, for example, are obtained. As is clear from the result of the measurement, the level of sound received by the left ear is lower than the level of sound received by the right ear, which is even true of low frequency sound that does not provide a directional feeling (especially a frequency range corresponding to the low frequency range signal SLFE).
In addition, since the converting circuit 3F in the reproducing apparatus 200 of FIG. 3 is provided so that a sound field that would be obtained by speaker reproduction can be reproduced by means of headphones, the converting circuit 3F also reproduces such decreases in the level of the low frequency range of the signals SLF and SRF for front channels.
Thus, in the reproducing apparatus 200 of FIG. 3, even when the low frequency range signal SLFE is added to the audio signals SLF to SRB for respective channels by the adding circuits 21 to 24, the level of the added low frequency range signal SLFE is lowered by the converting circuit 3F. As a result, the level of low frequency sound reaching both ears of the listener is lowered.
FIG. 7 shows a result obtained when frequency characteristics of the converting circuit 3F are measured. The result of the measurement represents frequency characteristics of output signals of the converting circuit 3F obtained when a speaker SP60 is disposed at a position 60xc2x0 to the right of the front of a dummy head DHD, and the left ear and the right ear of the dummy head DHD receive sound reproduced by the speaker SP60, as shown in FIG. 5.
As is also clear from the result of the measurement, the converting circuit 3F lowers the level of low frequency ranges of the audio signals SLF and SRF. The same is true of the converting circuit 3B.
Thus, even when the low frequency range signal SLFE is added to the audio signals SLF to SRB by the adding circuits 21 to 24, the level of the added low frequency range signal SLFE is lowered by the converting circuits 3F and 3B. As a result, the level of low frequency sound reaching both ears of the listener is lowered.
In accordance with the above, the present invention is intended to prevent lack of low frequency sound to be obtained from an audio signal representing a component in a low frequency range when multichannel stereo reproduction is performed by means of headphones.
For example, an audio reproducing apparatus according to the present invention comprises a converting means for converting main audio signals included in an audio signal for a plurality of channels into audio signals that provide a reproduction sound field similar to that obtained in reproduction by means of speakers even when the audio signals are reproduced by means of headphones; and an adding means for adding an auxiliary audio signal included in the audio signal for a plurality of channels to an audio signal outputted from the converting means; wherein an audio signal outputted from the adding means is supplied to headphones.
Thus, an audio signal representing a component in a low frequency range is added to audio signals for respective channels without its level being lowered.